1. Field of the Invention
The present invention relates to an oxygen indicator indicating the presence or absence of oxygen in a gaseous atmosphere for maintaining foodstuffs, electronic parts, electrical products having electronic parts, metallic parts or products having metallic parts.
By the "oxygen indicator" in the specification and claims is meant an oxygen indicator composition, a tablet comprising an oxygen indicator composition and film or sheet on which an oxygen indicator composition is coated. The presence or substantial absence of oxygen in the gaseous atmosphere surrounding the indicator is indicated by reversible color change of the indicator.
2. Prior Art
U.S. Pat. Nos. 4,169,811 and 4,349,509 disclose an oxygen indicator containing a reducing agent glucose. However, water or moisture is necessary for these indicators to exhibit the function of the Indicator. When these indicators are used under anhydrous conditions, the function of the indicator is not sufficiently exhibited.
Even when water is previously added to the oxygen indicator in order to avoid the Inconvenience, the moisture is absorbed in a desiccant or a dried foodstuff, whereby the function of the indicator is reduced. In addition, the water in the indicator moistens the dried foodstuffs to be maintained in good state.
Japanese Patent Publication (Kokai) Nos. 61-144568 and 62-12853 disclose an oxygen indicator comprising a thiazine dyestuff and an organic acid salt of ethanol amine as essential components in order to overcome the above mentioned shortcoming. However, when the oxygen indicator is exposed to light, there is possibility that the indicator will show the color indicating the absence of oxygen in spite of the presence of small amount of oxygen.
In other words, prior oxygen indicators do not sufficiently function in the absence of moisture or when exposed to light.